"It is critical that we elect a Republican to the U.S. Senate in 2014 who will hold this Administration accountable, rein in wasteful spending and reduce our debt, and protect our liberties. I will work hard to help elect a South Dakota Republican to the U.S. Senate next year," she wrote.

In late May, Noem told The Hill that she was discussing a potential Senate run with groups interested in her candidacy, though she wouldn't elaborate.

She was considered a more conservative alternative to the only announced Republican in the race, former Gov. Mike Rounds.

But some Republicans were worried that if Noem launched a primary challenge against Rounds, that would ultimately weaken the eventual nominee and jeopardize Republican chances at the seat.

Noem's decision not to run eliminates a hurdle to Rounds's nomination, but outside conservative groups are still actively looking for a potential challenger.